Methods and devices for measuring a concentrated light beam

ABSTRACT

Methods and devices are provided for profiling a beam of light that includes a wavelength λ. The beam of light is received. Secondary light is generated at a wavelength λ′ different from wavelength λ by fluorescing a material with the received beam of light. The secondary light is separated from the received beam of light. The separated secondary light is optically directed to a sensor.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of, and claims the benefit of thefiling date of, U.S. Nonprovisional Pat. Appl. No. 11/261,439, entitled“METHODS AND DEVICES FOR MEASURING A CONCENTRATED LIGHT BEAM,” filedOct. 28, 2005, now allowed, which is a nonprovisional of, and claims thebenefit of the filing date of, U.S. Prov. Pat. Appl. No. 60/623,720,entitled “METHODS AND DEVICES FOR MEASURING A CONCENTRATED LIGHT BEAM,”filed Oct. 28, 2004 by Timothy N. Thomas, the entire disclosure ofwhich, including the Appendix, is incorporated herein by reference forall purposes. The Appendix to U.S. Prov. Pat. Appl. No. 60/623,720corresponds to published PCT application WO 03/089,184 and is sometimesreferred to herein as “the Thermal Flux Processing application.”

BACKGROUND OF THE INVENTION

Concentrated light beams, such as are provided by certain lasers, areused in a variety of different applications. One characteristic of suchbeams that makes them valuable in these varied applications is theirability to deliver a highly concentrated beam of optical power as acollimated beam that provides precision in position, size, anddistribution at high intensity levels. The quality of this performancemay, however, be impaired by degradation of the quality of the lightbeam, such as may result from aging of components, vibration and shock,deterioration of a lasing medium, thermal drift, poor optical alignment,and various other sources of component nonlinearity. A change in theintensity profile of the light beam, even if there is no change in thetotal power output of the beam, may have significant adverseconsequences on performance.

Because of these concerns, it is useful for the light beam to beprofiled periodically so that the intensity profile may be evaluated. Achallenge in performing such profiling is the intensity of the beamitself since the very high power transfer may damage the profilingdevice. In particular, many conventional beam-profiling systems facedifficulties when beam power density approach values on the order ofthousands of watts per square centimeter.

There is accordingly a general need in the art for methods and devicesthat permit profiling of concentrated light beams.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention make use of a fluorescent material thatradiates light at a different wavelength than the wavelength of thelight beam to be profiled, using this radiated light to evaluate thelight beam. In some embodiments, a method is thus provided for profilinga beam of light that includes a wavelength λ. The beam of light isreceived. Secondary light is generated at a wavelength λ′ different fromwavelength λ by fluorescing a material with the received beam of light.The secondary light is separated from the received beam of light. Theseparated secondary light is optically directed to a sensor.

In some embodiments, the fluorescent material is disposed within aportion of the received beam of light, and the beam of light is movedrelative to the fluorescent material. For example, the beam of light maybe incident on a cylinder having an axis substantially orthogonal to anincident direction of the beam of light so that the beam of light ismoved relative to the fluorescent material by rotating the cylinderabout the axis. In another instance, the beam of light is incident on adisk having an axis substantially parallel to an incident direction ofthe beam of light, with the beam of light being moved relative to thefluorescent material by rotating the disk about the axis; the sensor mayalso be rotated about the axis of the disk. The directed separatedsecondary light may be focused onto the sensor. In addition, thedirected separated secondary light may be filtered to block light atwavelength λ. In one embodiment, λ is approximately 808 nm and λ′ isapproximately 1064 nm. The beam of light may be substantiallymonochromatic in an embodiment.

In other embodiments, a device is provided for profiling a beam of lightthat includes a wavelength λ. The device comprises a body, a fluorescentmaterial, a light sensor, and an optical arrangement. The fluorescentmaterial is disposed proximate a surface of the body oriented to receivethe beam of light. The fluorescent material radiates at a wavelength λ′different from wavelength λ in response to excitation by the beam oflight, and the body is substantially transparent to wavelengths λ andλ′. The optical arrangement is adapted to separate the light atwavelength λ′ from the beam of light and to direct the light atwavelength λ′ to the light sensor.

The fluorescent material may be disposed on the surface of the body,such as in one embodiment where it is comprised by a film deposited overthe surface of the body, or it may be disposed within the body under thesurface of the body. The optical arrangement may include a surfacewithin the body that substantially transmits light having a wavelengthof one of λ and λ′ and substantially reflects light having a wavelengthof the other of λ and λ′. In one embodiment, the optical arrangementincludes a lens disposed to focus the light directed to the light sensoronto the light sensor. The optical arrangement may further include afilter having transmission characteristics that block transmission oflight having wavelength λ disposed to filter the light focused onto thelight sensor. In different embodiments, the light sensor may comprise aphotodetector or may comprise a camera. In one embodiment, λ<1000 nm andthe fluorescent material comprises Nd:YAG.

Different structures for the body may be accommodated. In oneembodiment, the body comprises a hollow cylinder having an axissubstantially orthogonal to an incident direction of the beam of light.The optical arrangement includes a surface within a hollow portion ofthe hollow cylinder that substantially transmits light having awavelength of one of λ and λ′ and substantially reflects light having awavelength of the other of λ and λ′. A motor coupled with the body mayrotate the hollow cylinder about the axis. In another embodiment, thebody comprises a disk having an axis substantially parallel to anincident direction of the beam of light. A motor coupled with the bodymay rotate the disk and the optical arrangement about the axis.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram summarizing methods of the invention in certainembodiments;

FIG. 2 is a schematic illustration of a device for measuring aconcentrated light beam in one embodiment;

FIG. 3 is a schematic illustration of a device for measuring aconcentrated light beam in another embodiment;

FIGS. 4A-4C are schematic side, end, and isomorphic-projection views ofa device for measuring a concentrated light beam in a furtherembodiment;

FIGS. 5A and 5B are schematic side and isomorphic-projection views of adevice for measuring a concentrated light beam in still anotherembodiment; and

FIG. 6 is a top view of a device for measuring a concentrated light beamin an additional embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention make use of a material that fluoresces inresponse to the energy flow provided by the concentrated beam of light.The invention was developed by the inventors during the course of theirwork on a thermal processing system like the one described in detail inthe Thermal Flux Processing application, but the invention is notlimited to such applications and may be used more generally in profilingconcentrated light beams that are used in other applications as well. Inthe thermal processing system described in the Thermal Flux Processingapplication, the concentrated light beam is provided by acontinuous-wave radiation source, and is collimated and focused by anoptical arrangement into a line of radiation extending across asubstrate surface as part of a process for semiconductor-devicemanufacture. Heat generated at the surface of the target by theconcentrated light raises the temperature to high values useful forannealing in a time frame short enough to prevent diffusion. Whileembodiments of the invention are suitable for use with continuous-wavelight sources like the one described in the Thermal Flux Processingapplication, other embodiments may be used to profile concentrated lightprovided as bursts, pulses, or flashes. Furthermore, while the opticalarrangement described in the Thermal Flux Processing application is usedto focus the light into a line, different embodiments of the inventionmay be used to profile other geometric configurations of concentratedlight.

In many embodiments, the concentrated beam of light is substantiallymonochromatic. For a specific application described in the Thermal FluxProcessing application in which a silicon substrate is used, theconcentrated light has a wavelength between about 190 nm and 950 nm,with a specific example of light having a wavelength of 808 nm beingdescribed. In some of the discussion below, this example is alsodiscussed for purposes of illustration, but the invention is not limitedto any particular wavelength for the concentrated beam of light.Furthermore, the invention is not limited to profiling of monochromaticbeams of light and embodiments may be applied to other beams that havestable spectra.

The fluorescent light generated by the interaction of the concentratedbeam of light with the fluorescent material has generally the sameintensity profile as the concentrated beam of light, but at asignificantly reduced overall intensity and generally at a differentwavelength. For example, in some embodiments, the fluorescent materialcomprises neodymium:(yttrium aluminum garnet) (“Nd:YAG”), which respondsto the 808-nm light beam by fluorescing at a wavelength of 1064 nm. Anoptical arrangement is used to separate the resulting combination of808-nm and 1064-nm light, directing the high-intensity 808-nm light sothat it is lost and directing the low-intensity 1064-nm light to asensor. The low-intensity 1064-nm light is profiled with the resultsobtained from the detector and used as an indicator of the profile ofthe high-intensity 808-nm light. Because the profiling is performed withlight of a lower intensity there is insignificant risk of heating themeasuring sensor to the point that it melts, evaporates, or is otherwisedamaged by the intensity of the light.

FIG. 1 provides a generalized overview of different embodiments of theinvention. At block 104, a concentrated, a light beam that includes awavelength λ is directed towards a profiling device. The light beam isused at block 108 to fluoresce secondary light from a fluorescent sourceat a different wavelength λ′. The combined light at wavelengths λ and λ′is directed towards a spectral separator at block 112 to separate thelight into its individual wavelength components. The optical structureof the profiling device causes the light at wavelength λ to be lost, asindicated at block 116, and causes the light at wavelength λ′ to bedirected to the sensor for measurement.

One specific embodiment for the profiling device is illustratedschematically in FIG. 2. The profiling device comprises a block 204 ofmaterial, such as glass, that is generally transparent to light atwavelengths λ and λ′. The block 204 may comprise an intermediate surface208 that is substantially reflective at one of the two wavelengths andsubstantially transmissive at the other of the two wavelengths. Forinstance, the optical block 204 may comprise halves of a rectangularprism that have been joined along their respective hypotenuses after oneof the surfaces has been coated with an optical coating having thedesired transmission/reflection characteristics. In the specificillustration, the optical coating is transmissive at wavelength λ, whichis the wavelength of the light beam 228 incident on the profilingdevice, and is reflective at wavelength λ′, which is the wavelength ofthe secondary light generated by the fluorescent source 224. In theembodiment of FIG. 2, the fluorescent source 224 is provided as adot-like structure, although a variety of different geometric patternsmay be used in alternative embodiments, including a line structure, aplurality of separated dot-like structures, a combination of line anddot-like structures, or other geometric patterns. The fluorescent source224 is positioned proximate a surface of the body 204 such that theincident light beam 228 encounters the fluorescent source 224 togenerate the secondary light. The fluorescent source 224 may be disposedon the surface of the body 204 or may be implanted below the surface ofthe body.

The secondary light 236 is focused onto a sensor 220 by another part ofthe optical arrangement. The specific embodiment shown in FIG. 2 uses alens 216 to focus the light, but any arrangement of lenses and/orreflective surfaces such as mirrors may be used to accomplish thefocusing. In some embodiments, an optical component designed to increasecollection of incoming rays, such as a Winston cone, may be used. Theoptical arrangement may also conveniently comprise a filter 212 that istransmissive at wavelength λ′ but opaque at wavelength λ along the pathof the secondary light 236 to ensure that no stray light from theinitial beam 228 is directed onto the sensor 220. While the filter 212is shown disposed to encounter the secondary light 236 prior to anencounter with the lens 216, the order of encounters may be reversed,with the secondary light 236 encountering the lens before it encountersthe filter 212. The sensor 220 may comprise a photodetector of the typeknown in the art. By moving the beam or the optical arrangement, such asby moving the prism block 204, the lens 216, the filter 212, and thesensor 220 in concert, the output of the sensor 220 is representative ofthe light intensity profile of the incident beam 228.

An alternative embodiment is illustrated in FIG. 3, and also uses ablock 304 of material that is substantially to light at wavelengths λand λ′, and includes an intermediate surface 308 substantiallyreflective to one of the wavelengths and substantially transmissive tothe other of the wavelengths. Instead of providing discrete sources oranother geometric pattern of fluorescent material, the block 304 iscoated with a film 320 of fluorescent material or is implanted with thefluorescent material. The profiling device otherwise functions similarlyto the description provided in connection with FIG. 2, with an incidentbeam of light 324 interacting with the film 320 of fluorescent materialto generate secondary light 332. The original light at wavelength λ andthe secondary light at wavelength λ′ are directed in differentdirections by interacting with the intermediate surface 308. As before,it does not matter which wavelength is reflected and which wavelength istransmitted, although FIG. 3 illustrates the specific case where λ′reflected and λ is transmitted. The secondary light is directed to asensor for imaging.

FIG. 3 also illustrates a further variant in the form of the sensor,which may be provided as a camera 316 capable of detecting light imagedover the surface of the block 304. In this way, the full incident beam324 may be profiled without moving the beam 324 or optical arrangement.A filter 312 that transmits light at wavelength λ′ but that issubstantially opaque at wavelength λ may conveniently be positionedalong the path of the secondary light to prevent stray light atwavelength λ from reaching the camera 316. The camera 316 may be acharge-coupled device or other type of camera in different embodiments.

Alternative arrangements for accomplishing the motion used to profilethe entire incident beam are illustrated in FIGS. 4A-5B, with FIGS.4A-4C illustrating one embodiment and FIGS. 5A-5B illustrating analternative embodiment. In each of these embodiments, the same generalstructure is illustrated for the portion of the optical arrangement thatfocuses the secondary light onto the detector. The structure is shown ascomprising a filter that transmits light of wavelength λ′ but not lightof wavelength λ, and a focusing lens, but may include additional oralternative optical components such as a Winston cone to increasecollection of light rays.

The embodiment illustrated in FIGS. 4A-4C is shown with a side view inFIG. 4A, an end view in FIG. 4B, and an isometric-projection view inFIG. 4C. This embodiment uses a cylinder 404 transparent at wavelengthsλ and λ′, with at least a portion of the optical arrangement disposedwithin the transparent cylinder 404. Positioning of the portion of theoptical arrangement disposed within the cylinder 404 is simplified whenthe cylinder 404 is provided as a hollow cylinder so that opticalcomponents may be placed within the hollow portion. The cylinder 404 iscoupled with a motor (not shown) configured to rotate the cylinder aboutan axis 440. Such rotation permits fluorescent material 408 disposed ina geometric pattern on a surface of the cylinder 404 to be moved intodifferent positions, with the rotational position being synchronizedwith the location of the fluorescent material. A beam of light 428 atwavelength λ incident on the cylinder 404 causes the fluorescentmaterial to emit secondary light 436 at wavelength λ′. The combinedlight is separated with an optical component 424 that directs thesecondary light to be directed towards the sensor 420, being focusedwith a lens 416 and perhaps additionally filtered by a filter 412 asdescribed above in connection with other embodiments. The separation ofthe combined light may be accomplished by using a structure similar tothat described above. In particular, optical component 424 may comprisea block of material that is transparent at wavelength λ, covered by acoating that reflects light at wavelength λ′ and transmits light atwavelength λ. In alternative embodiments, the reflective andtransmissive properties may be reversed so that the secondary light atwavelength λ′ is transmitted and then directed to a portion of theoptical arrangement that focuses and senses the light.

A further variant is illustrated in FIGS. 5A and 5B, which respectivelyprovide a side and isomorphic-projection view of an embodiment that usesa rotating disk 504. The disk 504 is formed of a material that istransparent to light at wavelengths λ and λ′ and includes fluorescentmaterial 508 disposed on a surface of the disk 504 in a geometricpattern like those described above. The optical arrangement is similarto that described in connection with FIGS. 4A-4C, with an opticalcomponent 512 being provided to reflect light of one of the wavelengthsλ and λ′ and to transmit light of the other wavelength. A beam of light528 incident on the disk 504 causes the fluorescent material 508 togenerate secondary light 536 that is thereby focused onto a sensor 524with a lens 520, perhaps after being filtered by a filter 516 to preventstray light from reaching the sensor 524. The disk 404 and opticalarrangement are coupled with a motor (not shown) that causes rotationabout axis 540 so that different portions of the incident beam 528 maysuccessively be imaged by the sensor 524.

A further alternative is illustrated with FIG. 6, which provides a topview of another embodiment that uses a rotating disk 604. A plurality ofdots 612 of fluorescent material are distributed about the disk in aspiral pattern so that as the disk 604 rotates about axis 616, differentones of the dots 612 are exposed to the beam 608 at different radialdistances from the axis 616. In some embodiments, the disk 604 isrotated rapidly so that each of the dots 612 is exposed repeatedly tothe beam 608 but for a relatively short period of time. In suchinstances, a single detector may be used to collect the secondary light,and cross talk between pixels may be minimized. An additional advantagethat results from rapidly spinning the disk is that any heat adsorbedfrom the beam is distributed over a relatively large volume, which isrelated to the duty cycle of the dots, thereby increasing the maximumusable power density. For purposes of illustration, the inventors havecalculated that a disk having a diameter of 12 cm may yield about a 4-μmresolution in the slow axis and dot-size limited in the fast axis.

The specific embodiments described above are intended to be illustrativeof different aspects of the invention, and there are a number ofalternatives that may be used, particularly in separating the combined λand λ′ light and in directing the secondary light to the sensor. Forexample, the separation of the light has been described in each of theembodiments with use of a coating that transmits light at one of thewavelengths and reflects light at the other of the wavelengths. Such astructure has the advantage that substantially the entire strength ofthe secondary light is retained and directed to the sensor, particularlywhen the optical arrangement also comprises a component designed toimprove light collection such as a Winston cone. In other embodiments,however, other techniques for separating substantially dichromatic lightmay be used, even if such techniques result in some loss in intensity ofthe secondary light. For example, an arrangement in which thedichromatic light is initially focused and directed to a splitter, withone output of the splitter being further directed to a filter thatpasses only light of wavelength λ′ could be used to collect thesecondary light. This and other similar arrangements may be combinedwith the structures otherwise described in connection with FIGS. 2-6.

Furthermore, in other alternative embodiments, the use of a fluorescentmaterial may be avoided by providing scattering features that act toscatter light at the wavelength λ of the incident beam, therebysignificantly reducing its intensity so that it may be sampled by thesensor. Such scattering features may be placed on the surface of, orembedded with, the transparent structures 204, 304, 404, and 504described above. The detection of light may then be performed withoutincluding a filter that blocks transmission at the wavelength λ of theincident light.

Having described several embodiments, it will be recognized by those ofskill in the art that further modifications, alternative constructions,and equivalents may be used without departing from the spirit of theinvention. Accordingly, the above description should not be taken aslimiting the scope of the invention, which is defined in the followingclaims.

1. A method for profiling a beam of light that includes a wavelength λ,the method comprising: rotating a disk about a central axis of the disk,wherein the disk includes a series of discrete fluorescent dots disposedin a spiral pattern around the disk; directing a beam of light with awavelength λ toward the rotating disc, such that the beam of light isincident on the disk along a path substantially parallel with thecentral axis of the disk; separating secondary light from the receivedbeam of light, the secondary light having been generated by fluorescingthe fluorescent dots with the received beam of light and the secondarylight having a wavelength λ′ different from the wavelength λ; andoptically directing the separated secondary light toward a sensor. 2.The method recited in claim 1 further comprising focusing the directedseparated secondary light onto the sensor.
 3. The method recited inclaim 1 further comprising filtering the directed separated secondarylight to block light at wavelength λ.
 4. The method recited in claim 1wherein λ is approximately 808 nm and λ′ is approximately 1064 nm. 5.The method recited in claim 1 wherein the beam of light is substantiallymonochromatic. 6 . A device for profiling a beam of light that includesa wavelength λ, the device comprising: a prism; a film of fluorescentmaterial disposed over a surface of the prism oriented to receive thebeam of light, wherein: the fluorescent material radiates light at awavelength λ′ different from wavelength λ in response to excitation bythe beam of light; and the prism is substantially transparent towavelengths λ and λ′; a light sensor; and an optical arrangementconfigured to separate the light at wavelength λ′ from the beam of lightand to direct the light at wavelength λ′ to the light sensor.
 7. Thedevice recited in claim 6 wherein the optical arrangement includes asurface within the prism that substantially transmits light having awavelength of one of λ and λ′ and substantially reflects light having awavelength of the other of λ and λ′.
 8. The device recited in claim 6wherein the optical arrangement includes a lens disposed to focus thelight directed to the light sensor onto the light sensor.
 9. The devicerecited in claim 8 wherein the optical arrangement further includes afilter having transmission characteristics that block transmission oflight having wavelength λ disposed to filter the light focused onto thelight sensor.
 10. The device recited in claim 6 wherein the light sensorcomprises a photodetector.
 11. The device recited in claim 6 wherein thelight sensor comprises a camera.
 12. The device recited in claim 6wherein wavelength λ is less than 1064 nm and the fluorescent materialcomprises Nd:YAG.
 13. The device recited in claim 6 wherein wavelength λis less than 1064 nm and the fluorescent material comprises Nd ionsembedded in a glass.
 14. The device recited in claim 6 wherein the beamof light is substantially monochromatic.
 15. A device for profiling abeam of light that includes light with a wavelength λ, the devicecomprising: a disk rotatable about a central axis of the disk, the diskbeing oriented to receive the beam of light; a plurality of fluorescentdots embedded within the rotatable disk in a spiral pattern around thecentral axis of the disk, wherein the fluorescent dots radiate light ata wavelength λ′ different from wavelength λ in response to excitation bya beam of light of wavelength λ; a light detector; and light directingmeans for separating light with wavelength λ′ from light with wavelengthλ and for directing the light with wavelength λ′ toward the lightdetector.
 16. The device for profiling a beam of light according toclaim 15 wherein the light directing means includes an optical elementwith a coating that transmits light with wavelength λ and reflects lightwith wavelength λ′.
 17. The device for profiling a beam of lightaccording to claim 15 wherein the light directing means includes anoptical element with a coating that transmits light with wavelength λ′and reflects light with wavelength λ.
 18. The device for profiling abeam of light according to claim 15 wherein the light directing meansincludes a Winston cone.
 19. The device for profiling a beam of lightaccording to claim 15 wherein the light directing means includes afilter having transmission characteristics that block transmission oflight having wavelength λ.
 20. The device for profiling a beam of lightaccording to claim 15 wherein the fluorescent dots comprise Nd:YAG.